1. Field of The Invention
The invention relates to a process for preparing nicotinic acid from solutions of ammonium nicotinate.
2. Background Art
A known process for preparing nicotinic acid is based on the oxidation of 3-methylpyridine (xcex2-picoline) with atmospheric oxygen in the presence of water with heterogeneous catalysis. However, such process has the disadvantage that, in a side reaction, some of the 3-methylpyridine is broken down in a sequence of oxidation and hydrolysis steps to form ammonia, which, together with the main product nicotinic acid, forms ammonium nicotinate. The latter, in contrast to the free nicotinic acid, is highly water soluble and thus makes the work-up of the product mixture more difficult, and furthermore it represents a loss in yield, if it cannot be converted back to nicotinic acid. The latter is possible, for example, by adding a strong acid, but this forms the ammonium salt of this acid, which must be separated off and disposed of as waste. In addition, it must be noted here that nicotinic acid, as a pyridine derivative, can also react as a base and, with an excess of the strong acids, can form a salt.
The object of the invention is to provide a process which permits the transformation of ammonium nicotinate to nicotinic acid without adding auxiliaries and without producing wastes. According to the invention this object is achieved by the process according to the invention.
It has surprisingly been found that an aqueous solution of ammonium nicotinate can be converted to nicotinic acid by spray-drying. In this process, the ammonia, together with the water, escapes in the exhaust gas of the spray-dryer,
The spray-drying is preferably carried out at a drying gas temperature (at the inlet) of from 160xc2x0 to 250xc2x0 C. A suitable drying gas is air or an inert gas, such as, nitrogen or argon. The outlet temperature is advantageously kept below 110xc2x0 C. in order to prevent sublimation of the product.
Preferably, the spray-drying is carried out in a fluidized-bed spray-dryer. Fluidized-bed spray-dryers of this type are available under the name FSD(trademark), for example, from Niro A/S in DK-2860 Sxc3x8borg, Denmark.
The nicotinic acid obtained after the spray-drying still contains small amounts of ammonium nicotinate, depending on the drying temperature. It has been found that this can be further decreased by a thermal post-treatment in a fluidized bed at from 100xc2x0 to 200xc2x0 C., preferably from 130xc2x0 to 170xc2x0 C. Any nicotinic acid dust produced in this treatment can be recirculated to the spray-dryer.
As an alternative to this post-treatment in the fluidized bed, post-treatment under reduced pressure (partial vacuum or vacuum) at relatively low temperature can also be carried out. The pressure employed in this case is advantageously below 100 mbar, preferably below 50 mbar. The temperature in this case is expediently from 70xc2x0 to 150xc2x0 C., preferably from 80xc2x0 to 120xc2x0 C. Particularly good results have been achieved at from 10 to 15 mbar, from 80xc2x0 to 90xc2x0 C. and a treatment time of from xc2xd to 1 hour. At this low temperature, the losses due to sublimation are minimal and a product having excellent transparency, that is, no discoloration, is obtained.
Preferably, the process according to the invention is carried out using an ammonium nicotinate solution which was obtained by adding ammonia to the, possibly concentrated, aqueous crude solution from the catalytic oxidation of 3-methylpyridine. This can be achieved, for example, by metering gaseous or aqueous ammonia into an absorption column, to which is fed the gaseous reaction mixture from the oxidation reactor and from which the ammonium nicotinate solution is taken off as bottom product and the excess water is taken off overhead.
The ammonia needed to prepare the ammonium nicotinate solution is in this case preferably wholly or partly withdrawn from the spray-drying exhaust gas. For this purpose, for example, the dryer exhaust gas can be cooled below the dew point and the condensing ammonia water can be separated off and recirculated to the nicotinic acid absorption. Since, as mentioned above, small amounts of ammonia are produced in any case as a by-product in the oxidation of 3-methylpyridine, overall an ammonia excess is produced, so that with sufficient efficiency of the ammonia recycling, in the continuous operation, feed of external ammonia can be dispensed with completely.
Likewise, the water present in the spray-drying exhaust gas is preferably completely or partially recirculated to the oxidation reactor. Since, in the oxidation of 1 mol of 3-methylpyridine to nicotinic acid, in the ideal case, 1 mol of water is produced, a small excess of water is produced, which must be discharged from the plant in a suitable manner. Similarly, any unreacted 3-methylpyridine present is advantageously recycled to the oxidation reactor. Furthermore, it is possible to feed pure oxygen instead of air as oxidizing agent in the steady state to a continuous plant for carrying out the process according to the invention and to circulate the drying gas, so that a plant with minimum production of exhaust gas results.
An advantage of the process according to the invention is also that the nicotinic acid thus obtainable is free-flowing without additional treatment and virtually has no tendency to clumping even at high temperature and relative humidity. Furthermore, by varying the spray-drying operating parameters the particle size of the product can be set to the desired value.